Power devices are widely used to carry large currents and support high voltages. Modern power devices are generally fabricated from monocrystalline silicon semiconductor material. One type of power device is the thyristor. A thyristor is a bistable power semiconductor device that can be switched from an off-state to an on-state, or vice versa. Power semiconductor devices, such as thyristors, high-power bipolar junction transistors (“HPBJT”), or power metal oxide semiconductor field effect transistors (“MOSFET”), are semiconductor devices capable of controlling or passing large amounts of current and blocking high voltages.
Silicon bipolar transistors have, conventionally, been used for high power applications in motor drive circuits, appliance controls, robotics and lighting ballasts. This is because bipolar transistors can be designed to handle relatively large current densities, for example, in the range of 200 to 50 A/cm2 and support relatively high blocking voltages in the range of 500-2500V.
Despite the attractive power ratings achieved by bipolar transistors, there exist several fundamental drawbacks to their suitability for all high power applications. Bipolar transistors are current controlled devices that typically require relatively large base control currents, typically one fifth to one tenth of the collector current, to maintain the transistor in an on-state mode. Proportionally larger base currents can be expected for applications that also require high speed turn-off. Because of the large base current demands, the base drive circuitry for controlling turn-on and turn-off is relatively complex and expensive. Bipolar transistors may also be vulnerable to premature breakdown if a high current and high voltage are simultaneously applied to the device, as commonly required in inductive power circuit applications. Furthermore, it may be relatively difficult to operate bipolar transistors in parallel because current diversion to a single transistor typically occurs at high temperatures, making emitter ballasting schemes necessary. This current diversion generally results from the decrease in on-state voltage drop across the bipolar device with further increases in operating temperature.
Silicon power MOSFETs address this base drive problem. In a power MOSFET, the gate electrode provides turn-on and turn-off control upon the application of an appropriate gate bias. For example, turn-on in an n-type enhancement MOSFET occurs when a conductive n-type inversion layer is formed in the p-type channel region in response to the application of a positive gate bias. The inversion layer electrically connects the n-type source and drain regions and allows for majority carrier conduction between source and drain.
The power MOSFET's gate electrode is separated from the conducting channel region by an intervening insulating layer, typically silicon dioxide. Because the gate is insulated from the channel region, little gate current is required to maintain the MOSFET in a conductive state or to switch the MOSFET from an on-state to an off-state or vice-versa. The gate current is kept small during switching because the gate forms a capacitor with the MOSFET's channel region. Thus, only charging and discharging current (“displacement current”) is typically required during switching. Because of the high input impedance associated with the insulated-gate electrode, minimal current demands are placed on the gate and the gate drive circuitry can be easily implemented.
Moreover, because current conduction in the MOSFET occurs through majority carrier transport only, the delay associated with the recombination of excess minority carriers is not present. Accordingly, the switching speed of power MOSFETs can be made orders of magnitude faster than that of bipolar transistors. Unlike bipolar transistors, power MOSFETs can be designed to simultaneously withstand high current densities and the application of high voltages for relatively long durations, without encountering the destructive failure mechanism known as “second breakdown.” Power MOSFETs can also easily be paralleled, because the forward voltage drop of power MOSFETs increases with increasing temperature, thereby promoting an even current distribution in parallel connected devices.
The above-described beneficial characteristics of power MOSFETs are typically offset, however, by the relatively high on-resistance of the MOSFET's drift region for high voltage devices, which arises from the absence of minority carrier injection. As a result, a MOSFET's operating forward current density is typically limited to relatively low values, for example, in the range of 40-50 A/cm2, for a 600 V device, as compared to 100-120 A/cm2 for the bipolar transistor for a similar on-state voltage drop.
On the basis of these features of power bipolar transistors and MOSFET devices, devices embodying a combination of bipolar current conduction with MOS-controlled current flow were developed and found to provide significant advantages in certain applications over single technologies such as bipolar or MOSFET alone. One example of a device which combines bipolar and MOS characteristics is the Insulated Gate Bipolar Transistor (IGBT).
The IGBT combines the high impedance gate of the power MOSFET with the small on-state conduction losses of the power bipolar transistor. Because of these features, the IGBT has been used extensively in inductive switching circuits, such as those required for motor control applications. These applications require devices having wide forward-biased safe-operating-area (FBSOA) and wide reverse-biased safe-operating-area (RBSOA).
Silicon carbide (SiC) devices have also been proposed and used as power devices. Such devices include power MOSFETs such as are described in U.S. Pat. No. 5,506,421. Similarly, silicon carbide Junction Field Effect Transistors (JFETs) and Metal-Semiconductor Field Effect Transistors (MESFETs) have also been proposed for high power applications. See U.S. Pat. Nos. 5,264,713 and 5,270,554.
Silicon carbide IGBTs have also been described in U.S. Pat. Nos. 5,831,288 and 6,121,633, the disclosures of which are incorporated herein as if set forth in their entirety.
Notwithstanding the potential advantages of silicon carbide, it may be difficult to fabricate power devices, including IGBTs, in silicon carbide. For example, these high voltage devices are typically formed using a lightly doped epitaxial layer (n or p type) on a highly doped n-type conductivity silicon carbide substrate having a thickness of from about 300 to about 400 μm. Low resistivity p-type silicon carbide substrates may not be available as a result of the available acceptor species (Aluminum and Boron) having deep energy levels that may result in carrier freeze out. Thus, the exclusive use of n-type substrates may limit the polarity of available high voltage devices. For example, only p-channel Insulated Gate Bipolar Transistors (IGBTs) may be available. In addition, the available devices may only be capable of blocking voltages in one direction.
In conventional power circuits it is desirable to have a device that may have the control voltage applied to the device to turn the device on and off referenced to ground rather than to a high positive voltage level. However, to provide an IGBT where the gate is referenced to the grounded emitter of the device generally requires a highly doped p-type substrate. As is noted above, highly doped p-type substrates currently are more difficult to fabricate than n-type substrates in silicon carbide. With an n-type substrate a conventional silicon carbide IGBT would have its gate voltage referenced to the collector voltage that, in a typical power circuit, would be to a line voltage. Thus, present silicon carbide IGBTs may require more complex gate drive circuitry with level shifting components and result in more complex power circuits as a result of the structure of IGBTs, the electrical characteristics of silicon carbide and the limitations in fabrication of highly doped p-type silicon carbide substrates.
Furthermore, in order to form a blocking junction at the substrate-epitaxial layer interface, a planar edge termination structure may be formed or an edge beveling process may be used to reduce the likelihood of premature breakdown at the edges of the device. Forming planar edge termination structures on a backside of the device may be difficult and costly to implement as extensive processing may be needed after removal of the 300 to 400 μm thick n-type substrate. Edge beveling may include etching through the substrate or grinding/polishing the sidewalls of the device, which may also be difficult because the voltage blocking epitaxial layers are generally much thinner than the substrate.